Kosomot (Kosmoko or Kosomok)was an early version of Gothmog, Lord of the Balrogs. Originally his mother was a monster by the name of Ulbandi, and his father was Melkor.

The later change from Kosomot to Gothmog related to the development of Elvish from its primordial forms as well as the maturing attitude of Tolkien's faerytales.

Please note:] The name Kosomot / Kosmoko / Kosomok is only known from HoME, and therefore cannot be considered utterly canonical.

"Gothmog 'was a son of Melko and the ogress Fluithuin and his name is Strife-and-hatred, and he was Captain of the Balrogs and lord of Melko's hosts ere fair Ecthelion slew him at the taking of Gondolin. The Eldar named him Kosmoko or Kosomok(o), but 'tis a name that fitteth their tongue no way and has an ill sound even in our own rougher speech, said Elfrith [emended from Elfriniel].' (In a list of names of the Valar associated with the tale of The Coming of the Valar (I. 93) it is said that Melko had a son 'by Ulbandi' called Kosomot; the early 'Qenya' dictionary gives Kosomoko = Gnomish Gothmog, I.258." BoLT2, Fall of Gondolin - Commentary ~2 'Entries in the Name-list'